CFTR Functions as a Bicarbonate Channel in Pancreatic Duct Cells

Abstract

Pancreatic duct epithelium secretes a HCO3−-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO3− permeability of CFTR provides a pathway for apical HCO3− efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO3− induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pHi) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K+ concentration. Apical HCO3− fluxes activated by cyclic AMP were independent of Cl− and luminal Na+, and substantially inhibited by the CFTR blocker, CFTRinh-172. Furthermore, comparable HCO3− fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (ΔF) mice. To estimate the HCO3− permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO3− and 24 mM Cl− in the presence of 5% CO2. From the changes in pHi, membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO3− across the apical membrane were determined and used to calculate the HCO3− permeability. Our estimate of ∼0.1 µm sec−1 for the apical HCO3− permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO3− secretion. This suggests that CFTR functions as a HCO3− channel in pancreatic duct cells, and that it provides a significant pathway for HCO3− transport across the apical membrane.

SLC26 anion exchangers of guinea pig pancreatic duct: molecular cloning and functional characterization

“…However, voltagedependent HCO 3 Ϫ fluxes measured across the apical membrane (20). The DIDS-sensitive Cl Ϫ / HCO 3 Ϫ exchange activated in CFPAC-1 cells transfected with CFTR was also insensitive to changes in membrane potential, consistent with electroneutral anion exchange (39).…”

“…This alkaline fluid prevents premature intraductal activation of the digestive proenzymes secreted by pancreatic acinar cells and delivers them safely to the intestinal lumen, where the activated enzymes facilitate nutrient absorption by enterocytes (34,49). Secretin stimulates HCO 3 Ϫ secretion through activation of adenylate cyclase, with subsequent activation of HCO 3 Ϫ transport across the ductal epithelium apical membrane via the cystic fibrosis transmembrane regulator (CFTR) Cl Ϫ channel and Slc26 Cl Ϫ /HCO 3 Ϫ exchanger(s) (20,48). Slc26a3, Slc26a6, and Slc26a11 have been detected in the human and mouse pancreatic duct (16,19,30), but their respective contributions to pancreatic HCO 3 Ϫ secretion remain unclear.…”

“…In addition, numerous experiments have not detected electrogenicity of Cl Ϫ /HCO 3 Ϫ exchange mediated by Slc26a6 and Slc26a3 (1,9,17,26,29,31,59). Furthermore, in some conditions, the activity of CFTR alone may account for 50% or more of stimulated HCO 3 Ϫ secretion (20). We recently reported that stimulated HCO 3 Ϫ secretion across the apical membrane of guinea pig pancreatic interlobular duct reflects predominantly diisothiocyanatostilbene-2,2=-disulfonic acid (DIDS)-sensitive (Slc26a6-like) Cl Ϫ /HCO 3 Ϫ exchange functionally coupled with CFTR, operating in parallel with lower-activity, DIDS-insensitive (Slc26a3-like) Cl Ϫ /HCO 3 Ϫ exchange (53).…”

The CFTR Ion Channel: Gating, Regulation, and Anion Permeation

“…Chloride is an important CFTR substrate in tissues that mediate salt and fluid transport such as exocrine glands (Wine and Joo 2004). CFTRmediated bicarbonate transport also is relevant in pancreatic ducts and the airways (Ishiguro et al 2009;Kim and Steward 2009). …”

Mechanisms of Bicarbonate Secretion: Lessons from the Airways

“…It is generally accepted that the difference in V ap between normal and CF cells is due to the loss of CFTR channels and the higher Na þ permeability in CF cells (Boucher et al 1986(Boucher et al , 1988Boucher 1994a,b). Besides Cl 2 , CFTR also conducts HCO 3 2 , as shown in the studies of Gray et al (1990) and Linsdell and coworkers (Linsdell et al 1997;Tang et al 2009;Li et al 2011) and more recently by Ishiguro et al (2009). Therefore, normal airway cells can secrete HCO 3 2 through CFTR, whereas CF cells show a HCO 3 2 impermeability.…”